In essentially every process in which a mixture is prepared for a particular purpose, the constituents of that mixture usually need to be present in particularly proportioned amounts in order for the mixture to be effective for its intended use. In the aforementioned related patents and patent applications, the underlying objective is to reduce the amount of organic solvent present in a coating composition by the use of supercritical fluid, particularly, carbon dioxide. Understandably, with this objective in mind, it is generally desirable to utilize as much supercritical fluid as possible while still retaining the ability to effectively spray the liquid mixture of coating composition and supercritical fluid and also obtain a desirable coating on the substrate. Accordingly, here too, it is particularly preferred that there be prescribed proportionated amounts of supercritical fluid and of coating composition present in the liquid admixed coating formulation to be sprayed.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the coating composition. This practical upper limit is generally recognizable when the admixture containing coating composition and supercritical fluid breaks down from one phase into two fluid phases.
To better understand this phenomenon, reference is made to the phase diagram in FIG. 1 wherein the supercritical fluid is supercritical carbon dioxide fluid. In FIG. 1, the vertices of the triangular diagram represent the pure components of an admixed coating formulation which for the purpose of this discussion contains no water. Vertex A is solvent, vertex B is carbon dioxide and vertex C represents a polymeric material. It can be clearly seen in this Figure that the polymer and the solvent are completely miscible in all proportions, that the carbon dioxide and the solvent are likewise completely miscible in all portions, but that the carbon dioxide and the polymer are not miscible in any portion, and as such the carbon dioxide is a non-solvent for the polymer. The curved line BFC represents the phase boundary between one phase and two phases. The point D represents a possible composition of a coating composition in which supercritical carbon dioxide has not been added. The point E represents a possible composition of an admixed coating formulation after admixture with supercritical carbon dioxide. The addition of supercritical carbon dioxide fluid has reduced the viscosity of the viscous coating composition to a range where it can be readily atomized by passing it through an orifice such as in an airless spray gun. After atomization, a majority of the carbon dioxide vaporizes, leaving substantially the composition of the original viscous coating composition. Upon contacting the substrate, the remaining liquid mixture of the polymer and solvent component(s) will flow, i.e., coalesce, to produce a uniform, smooth film on the substrate. The film forming pathway is illustrated in FIG. 1 by the line segments EE'D (atomization and decompression) and DC (coalescence and film formation).
The amount of supercritical fluid, such as supercritical carbon dioxide, that can be mixed with a coating composition is generally a function of the miscibility of the supercritical fluid with the coating composition as can best be visualized by referring to FIG. 1.
As can be seen from the phase diagram, particularly as shown by arrow 100, as more and more supercritical carbon dioxide is added to the coating formulation, the compositions of the liquid admixed coating mixture approaches the two-phase boundary represented by line BFC. If enough supercritical carbon dioxide is added, the two-phase region is reached and the composition correspondingly breaks down into two fluid phases. Sometimes, it may be desirable to admix an amount of supercritical fluid which is even beyond the two phase boundary. Generally, however, it is not preferable to go much beyond this two phase boundary for optimum spraying performance and/or coating formation.
In addition to avoiding the two-phase state of the supercritical fluid and the coating composition, proper proportionation is also desirable to provide optimum spraying conditions, such as, formation of desired admixed viscosity, formation of desired particle size, formation of desired sprayed fan shape, and the like.
Accordingly, in order to spray liquid admixed coating formulations containing supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intermittent or periodic on-demand basis, it is necessary to prepare such liquid admixed coating formulations in response to such spraying by accurately mixing a proportioned amount of the coating composition with the supercritical fluid. However, the compressibility of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure or temperature results in large changes in the density of the supercritical fluid.
The compressibility of the supercritical fluids causes the flow of these materials, through a conduit and/or pump, to fluctuate. As a result, when mixed with the coating composition, the proportion of supercritical fluid in the resulting admixed coating formulation also correspondingly fluctuates instead of being uniform and constant. Moreover, the compressibility of liquid carbon dioxide at ambient temperature is high enough to cause flow fluctuations to occur when using reciprocating pumps to pump and proportion the carbon dioxide with the coating composition to form the admixed coating formulation. This particularly occurs when the volume of liquid carbon dioxide in the flow path between the pump and the mixing point with the coating composition is too large. The fluctuation can be promoted or accentuated by any pressure variation that occurs during the reciprocating pump cycle.
The above-referred-to related patents and patent applications disclose apparatus for effectively supplying, feeding, measuring, proportionating, pressurizing, heating, and spraying an admixed coating formulation consisting of an admixture of a non-compressible coating composition comprised of a high concentration of one or more solid resins or polymers selected from a substantial list comprised of acrylics, amino, polyesters, alkyds; a variety of organic solvents, including water in some instances; suspended solids such as metallic flakes and other pigments; and a compressible supercritical fluid, such as supercritical carbon dioxide, as a viscosity reduction diluent.
Unexpectedly, however, operating problems were encountered when a nitrocellulose lacquer based coating composition was used with the methods and apparatus disclosed in the preferred embodiments of the aforementioned Applications. For reasons not fully understood with this coating composition, precipitation of solids occurred at the carbon dioxide injection and mixing point resulting in apparatus plugging.
After several runs with the nitrocellulose lacquer based coating composition, inspection of the apparatus revealed that the precipitate, in the form of a solid, partially to fully plugged the carbon dioxide feed injection point of a horizontally positioned 180.degree. mixing tee, followed by additional plugging through the accumulation of said solids in the downstream static mixer connected to the injection point device.
As used herein and as is conventionally used in the art, a "180.degree. mixing tee" is defined as a pipe or tubing tee in which two fluids are introduced opposing each other in the run of the tee with mixed flow exiting through the branch of the tee. On the other hand, a "90.degree. mixing tee" is defined as a pipe or tubing tee in which one of the fluids is introduced through the branch of the tee to mix with the primary flow in the run of the tee with the mixture exiting through the run of the tee.
Clearly, what is needed is a simple method and apparatus to introduce a non-solvent, such as supercritical carbon dioxide, into a fluid containing a dissolved solid, such as a polymer or resin, for example. The method and apparatus should be such as to prevent the deposition of solids and the possible consequential plugging at the mixing point, and in other downstream apparatus, from the saturation induced precipitation, for example, of polymer(s) and resin(s) in coating compositions and admixed coating formulations by supercritical carbon dioxide fluid acting as a precipitant, as the coating composition fluid and the supercritical fluid liquid are introduced into the apparatus and are mixed and commingled therein.
In particular, methods and apparatus are needed wherein saturation of highly crystalline character polymer(s) and resins(s) does not occur through the contacting of said material by bubbles, plugs or slugs of the non-solvent, such as supercritical fluids, such as carbon dioxide, or even from stratified or annular flow patterns of the same, thereby avoiding precipitation and adherence of said solids within the apparatus and, accordingly, preventing eventually plugging in the apparatus.
The problems recognized cannot be practically and economically solved using wholly conventionally available devices.
The aforementioned U.S. Pat. No. 5,105,843 discloses a method and apparatus wherein a supercritical fluid, such as carbon dioxide, which may be a non-solvent for solids contained in a coating composition, is supplied to an isocentric low turbulence mixing apparatus such that it is interjected as a core of fluid within a flowing viscous coating composition fluid stream, which contains a precipitable solid polymer or resin.
While the methods and apparatus disclosed in U.S. Pat. No. 5,105,843 have successfully prevented the precipitation of dissolved solids and, therefore, plugging of the injector and downstream apparatus, when operating under conditions in which the non-solvent, such as carbon dioxide, is injected into the coating formulation more or less continuously, a problem has been discovered when the apparatus is used for intermittent operation over an extended period of time. During periods between operation, such as when the coating composition is not being sprayed, or when the spray apparatus is shut down over night, it has been found that the admixed coating formulation flows into the tube through which the carbon dioxide is injected, because the carbon dioxide has very low viscosity and low density so that it is readily displaced from the tube. Also, the carbon dioxide left inside the tube when the flow is shut off tends to dissolve into the admixed coating formulation. When the spray apparatus is started up, the carbon dioxide flow ejects most of the admixed coating formulation from the tube, but the interior wall of the tube remains wetted with a film of coating material. Over time, as carbon dioxide flows along the film, solvent is lost to the carbon dioxide flow, which causes the dissolved polymer to precipitate onto the tube wall as a solid layer. When the apparatus is again shut down, this polymer layer is then wetted with more admixed coating formulation, which precipitates more polymer when the unit is started up again. Therefore, as this process is repeated over time the layer of precipitated polymer on the tube wall becomes thicker and thicker until it eventually plugs the tube so that the apparatus must be shut down and the injector cleaned out. The accumulation of polymer on the tube wall increases the carbon dioxide velocity through the tube, so that it no longer matches the velocity of the coating formulation at the interface between the two as they leave the injector. The accumulated polymer also disrupts the desired knife edge at the end of the tube. Therefore, the interfacial flow of the two fluids becomes less laminar and more turbulent over time, which causes polymer to eventually begin to precipitate in the mixer and downstream apparatus.
Clearly, what is needed is a simple method and apparatus to introduce a non-solvent, such as supercritical carbon dioxide, into a fluid containing a dissolved solid, such as a polymer or resin, wherein precipitation of the dissolved solid is prevented not only during continuous operation but during the shut down and start up cycle. Preferably, such a method and apparatus would substantially eliminate the presence of a separate non-solvent phase that is in contact with the fluid containing the dissolved solid or that is in contact with the admixture of the two fluids, thereby substantially eliminating the interface across which solvent is lost, which causes precipitation of the dissolved solid. Preferably, the apparatus would contain no surface that is wetted by the non-solvent during operation and which could become wetted by the fluid containing the dissolved solid, or the admixture of the two fluids, during non-operation.